1. Field of the Invention
This invention involves optical fibers based upon the silica glass system in which either the core, the cladding, or both, have an index of refraction different frm that of pure silica as a result of the addition of an appropriate dopant. The system employed in the disclosed optical fibers includes nonglass forming constituents, such as alumina or zirconia, as dopants.
2. Disclosures of Interest
Over the past decade the suggestion of Kao and Hockham (Proceedings of IEE, Vol. 113, No. 7, July 1966, p. 1151) that silica would constitute an appropriate transmission medium for low loss optical fibers has been widely realized. Optical fibers are now routinely fabricated on a commercial basis with losses less than 1 dB/km in at least part of the optical region of the spectrum, generally extending from 0.7 to 1.7 microns. The fibers comprise a core and a cladding, with the cladding having an index of refraction lower, at least in part, than that of an index of refraction associated with the core. Both multi-mode fibers and single-mode fibers are routinely fabricated using various design parameters, such as appropriate gradations in index of refraction to obtain desirable characteristics, such as high bandwidth.
Dopants which are used include germania, an index raising dopant, which is the principal and most widely used dopant, as well as other minor dopants, such as phosphorus, and other index raising dopant, and fluorine and boron, index lowering dopants. In addition, boron and phosphorus are used to improve processing characteristics, such as those associated with sintering. Recently, boron has been less widely used because of its infrared absorption band which appears in the spectral range of interest. Where long wavelength transmission is of interest, boron is usually replaced with fluorine, which, like boron, lowers the index of refraction of silica glass systems, but which has a longer wavelength infrared absorption band.
Germania, currently the principal dopant used in optical fibers to increase the index of refraction of silica, has been widely used over the past decade, almost to the exclusion of all other index raising dopants; this, despite the fact that germania has significant economic and technical drawbacks, including high cost, scarcity, a tendency to only partially react to form particulate material, and relatively high volatility with a resultant tendency to "burnoff" during high temperature processing. Furthermore, acceptable incorporation of germania in silica requires a high concentration of oxygen in the reaction atmosphere (many times stoichiometry) which in many processes results in an extended deposition region and a resultant deleterious taper in the deposit. Additionally, in order to remove trace amount of hydroxyl impurities, it is advantageous to have a high chlorine-to-oxygen ratio in the reaction environment; however, this tends to further degrade the efficiency of germania incorporation. Despite these drawbacks, germania remains the preferred index raising dopant, and essentially the only one in commercial use, primarily because no acceptable alternate dopant has been effectively developed.
The literature is replete with suggestions of various dopants for use in the fabrication of optical fibers. Generally, these disclosures are in the form of a listing of a large number of possible dopants with the suggestion that combinations of the dopants may also be used. (See, for example, U.S. Pat. No. 4,157,790, column 5, lines 1-6. "Many suitable materials have been used as a dopant alone or in combination with each other to increase the refractive index of fused silica. These include, but are not limited to titanium oxide, tantalum oxide, aluminum oxide, lanthanum oxide, phosphorus oxide and germanium oxide.") The use of alumina as a dopant has been specifically investigated (S. Kobayashi et al, Electronics Letters, Vol. 10, No. 20, p. 410, 1974). Such work, however, has generally involved low concentrations of alumina dopant. The use of higher concentrations of alumina generally results in devitrification (J. F. MacDowell and G. H. Beall, Journal of the American Ceramics Society, Vol. 52, p. 17, January 1969), and hence is discouraged.